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He has thrown his weight firmly behind Mr Corbyn, attacking Labour MPs who are trying to depose him and insisting he must stay on as leader.

His bizarre claims about MI5 come only weeks after he cited another online conspiracy theory on the BBC’s Marr Show, suggesting a London-based communications firm is secretly orchestrating the coup attempt against Mr Corbyn.

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In his latest interview Mr McCluskey said he had discovered recently that one of his former trade union colleagues had been an MI5 informant in the 1970s - and that he is certain the security services are active in left-wing politics today.

“Anybody who thinks that that isn’t happening doesn’t live in the same world that I live in,” Mr McCluskey said.

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“Do you think that there’s not all kinds of right-wingers who are not secretly able to disguise themselves and stir up trouble?

“I find it amazing if people think that isn’t happening.”

It follows Labour MPs reporting increasingly high levels of verbal abuse from Mr Corbyn’s supporters since they launched their attempted coup.

Jeremy Corbyn is facing opposition to her leadership (Image: Getty)

Former leadership hopeful Angela Eagle has been forced to cancel constituency surgeries after a brick was thrown through her window and she was sent death threats.

She accused Mr Corbyn of “stirring” hatred against his opponents and said the leader “needs to be held to account”.

And she was withering about Mr McCluskey’s theory that it is really MI5 behind the abuse.

“I’ve known Len for 40 years. His comments are over the top,” she said.

“These are serious issues. Rape threats, death threats and organised bullying are not something to be ignored or minimised.

Angela Eagle was withering about Mr McCluskey’s theory that it is really MI5 behind the abuse (Image: BBC)

“We need to unite the Labour movement so we can take on the Tories and heal the country, not continue to ignore the cancer of political violence that has been unleashed in the last few months.”

Other female Labour MPs have said they now receive death threats on a daily basis and have strongly criticised Mr Corbyn for not doing more to rein in his supporters.

On Thursday 44 female Labour MPs sent a joint letter to the Labour leader demanding he take swifter action.

“Rape threats, death threats, smashed cars and bricks through windows are disgusting and totally unacceptable in any situation,” they wrote.

“This is acknowledged by all factions, yet the simple words of condemnation offered in response are inadequate.

“We expect swift and tangible action against those who commit such acts.”

It came as Mr Corbyn himself was branded a “bully” and a “hypocrite” by one of his own House of Commons enforcers.

Labour whip Conor McGinn claimed that when he challenged the leader recently over his failure to engage working class voters, Mr Corbyn threatened to contact Mr McGinn’s father – a Sinn Fein councillor in Ireland – to bring him back into line.

Writing on Twitter, Mr McGinn mocked the Labour leader’s ‘kinder politics’ pledge and branded him “a hypocrite who tried to bully me by using my family against me.”

In a statement later Mr McGinn added: “Thankfully, others dissuaded Jeremy from taking this course of action. The call was not made, and it would not have been well received.”

Jeremy Corbyn's supporters say he did not make the threats (Image: Reuters)

But spokesman for the Labour leader insisted Mr McGinn’s claims were “untrue”.

He added: “Jeremy did not at any point threaten to call Conor’s dad, nor did he call him.”

Mr Corbyn flatly denied the claims and told Sky News: “I wish some of my colleagues would concentrate on political issues.

“I regret the language that has been used by all of them. I don’t do any abuse, I don’t do any bullying, I don’t allow it to be done in any of my campaigning teams.

“And I’m very surprised and actually very disappointed that they should say that because politics has to be about bringing people in and I think we have done that spectacularly well.”

He went on: “I have made it absolutely clear that harassment, abusive language has absolutely no place our political discourse.

“I receive plenty of abusive language - I don’t respond to it in any way because I don’t wish to demean myself of promote the person that is using that kind of abusive language.”